In order to reduce noises and clarify a voice, a gain of a specific frequency is adjusted. A certain conventional document discloses a technique for presuming a noise for each frequency and removing the background noise in order to make it easy to hear received voice. The noise is modeled by a straight line regarding the frequencies and using an inclination and intercept as parameters. The modeling is performed for each of a bandwidth from sound intermediate frequencies to low frequencies and a bandwidth of high frequencies.
Moreover, another conventional document discloses a technique for emphasizing a formant in order to make it easy to hear received voices. In this document, formant frequencies of input voices are analyzed and emphasized to enhance clearness of the received voices. The formant is a property concerning sounds in case of speaking a language.
It is possible to enhance easiness to hear by suppressing the background noises that are mixed into the received voices and emphasizing the formants of the received voices.
However, in addition to the background noises and the property of speaking, there is a factor that makes it difficult to hear reproduced voices of the voice signals.
In other words, the conventional techniques cannot sufficiently solve the difficulty to hear the reproduced voices.